An ultrahigh molecular weight ethylene polymer, which has a very high molecular weight corresponding to a viscosity average molecular weight (Mv) of at least 1,000,000, is excellent in the impact resistance, self-rubricating properties, abrasion resistance, weather resistance, chemical resistance, dimensional stability, etc., and has high physical properties equal to those of an engineering plastic. Accordingly, its application to lining materials, line members for the food industry, machine components, artificial joint, sports goods, microporous membranes, etc., by various forming methods, has been attempted.
However, an ultrahigh molecular weight ethylene polymer has very low fluidity when melted due to its high molecular weight, and can hardly be formed by melt extrusion like a common polyethylene having a molecular weight of from several tens of thousands to about 500,000. Accordingly, for an ultrahigh molecular weight polyethylene, a method of directly sintering a polymer powder obtained by polymerization, a method of compression molding the powder, a molding method by a ram extruder of extruding the powder while intermittently compressing the powder, or a method of extruding the powder as dispersed in a solvent or the like and then removing the solvent, may, for example, be conducted.
Among them, it is known that by the method of extruding the powder as dispersed in a solvent or the like and removing the solvent, a porous film can be produced by monoaxial and biaxial stretching before and after removal of the solvent. And, the ultrahigh molecular weight polyethylene microporous film thus obtained is expected to have excellent physical properties such as heat resistance, strength and impact resistance due to the high molecular weight of the ultrahigh molecular weight polyethylene. However, a ultrahigh molecular weight polyethylene produced by a Ziegler catalyst commercially available at present has a ratio (molecular weight distribution) of the weight average molecular weight (Mw) and the number average molecular weight (Mn) of higher than 4 and has a broad molecular weight distribution, and accordingly the molded product obtained from the polyethylene may not have sufficiently improved strength and heat resistance and does not necessarily have expected performance.
Further, an ultrahigh molecular weight ethylene polymer having a narrow molecular weight distribution produced by using a metallocene catalyst or a post-metallocene catalyst has been proposed (for example, Patent Documents 1 and 2).